The main techniques and approaches of biochemical cytology, resting largely on the combined use of cell fractionation, enzymology, biochemistry and electron microscopy, are applied to a detailed analysis of the subcellular events underlying the pathogeny of atherosclerosis. The experiments will be carried out on human arteries, obtained at autopsy or surgery, and on arteries from chickens. They will focus on all major organelles, with particular emphasis on lysosomes and peroxisomes which previous work on cholesterol-fed rabbits has shown to be markedly altered in atheromatous arteries. Special attention will be paid to lysosomal cholesteryl esterase and other lipid processing enzymes to further test the hypothesis that lipids may accumulate in the lysosomes of diseased arterial smooth muscle cells as a result of a relative enzyme deficiency.